1. Field of the Invention
The present invention relates to an apparatus and a method for treating wafer substrates and the like by means of a reaction was, used in processes for manufacturing semiconductor devices, circuit boards for driving liquid crystals, etc., and more particularly, to an apparatus and a method of this type provided with a mechanism for reducing the amount of compounds adhering to the inner surface of a reaction vessel.
2. Description of the Related Art
Treatment apparatuses are widely used in processes for manufacturing semiconductor devices, circuit boards for driving liquid crystals, etc. In these apparatuses, a reaction gas is caused to flow into a reaction vessel in order to subject objects to be treated, such as wafers and circuit boards, to various treatments, including film formation, diffusion, oxidation, etching, etc.
According to these conventional apparatuses, however, a compound of the same composition as a target product may possibly adhere to the inner surface of the reaction vessel. The compound on the inner surface of the vessel may react again during chemical reaction of the product, so that it chemically contaminates the desired product, or is entrapped in the form of particulates in the product, thereby considerably reducing the quality of the product.
If excitation energy is externally supplied to hasten the chemical reaction, the compound on the vessel absorbs or reflects the energy necessary for the reaction. Accordingly, constant energy cannot be supplied to the objects to be treated in the vessel, so that it is hard to control the chemical reaction accurately.
In a conventional decompression CVD apparatus, for example, a silicon oxide film (e.g., SiO.sub.2) is formed in the following manner. First, a quartz-glass boat, which carries a plurality of wafers in regular arrangement, is inserted into a cylindrical quartz-glass reaction tube surrounded by a heating device, such as a resistance heater, through an open end of the tube. Thus, the boat is situated in a predetermined position in the reaction tube. Then, after the open end of the tube is closed airtight by means of, e.g., a disk-shaped stainlesssteel lid, the gases inside the reaction tube are exhausted to a predetermined pressure level. The inside of the reaction tube is kept at a temperature of, e.g., 700.degree. C. by means of the heater, and reaction gases, including silicon tetrachloride (SiCl.sub.4), oxygen (O.sub.2), etc., are mixedly introduced into the reaction tube, and also subjected to heat treatment.
If the aforesaid reaction is repeated, however, a considerable amount of silicon oxide particles adhere to the inner surface of the reaction tube, especially that portion of the inner surface near the region for the introduction of the reaction gas. Accordingly, the floating oxide particles adhere to the wafer surface, and foreign matter contaminates the silicon oxide film. If a contamination layer formed of the reaction compound, such as the oxide particles on the inner surface of the reaction tube, becomes thicker, the transmission of radiant heat, as a factor of energy supply from the heating device, is reduced, and the distribution of temperature inside the reaction tube deteriorates. Conventionally, therefore, the reaction tube with the compound thereon is periodically cleaned. This cleaning work requires processes for taking out the reaction tube from the CVD apparatus, removing or washing out the adhering compound by means of an exclusive-use cleaning machine, using hydrofluoric acid and pure water, drying the tube by means of a drying machine, attaching the cleaned reaction tube again to the apparatus, and effecting adjustment work. The cleaning process involves the removal and attachment of the reaction tube, and assembly and disassembly of a vacuum exhaust system, especially in the case of the decompression CVD apparatus, thus requiring much time and labor. Since the operation of the apparatus must be stopped during such work, moreover, the operating efficiency of the apparatus is lowered.
Disclosed in Japanese Patent Disclosure No. 53-52356 is a method in which an etching gas is caused to flow in a layer along the inner surface of a reaction tube, or a reaction gas is doped with a suitable amount of etching gas, thereby reducing deposition of an undesired reaction compound on the inner surface of the reaction tube. According to this method, however, the etching gas freely enters into a reaction region for substrates to be treated, thus adversely affecting the desired reaction on the substrates, in both physical and chemical aspects.
Disclosed in Japanese Patent Disclosure No. 61-176113 is a method in which an etching gas or solution, pure water, etc., are introduced successively into a reaction tube after heat treatment, and the tube is cleaned inside the housing of the heat treatment apparatus. According to this method, however, large-scale equipment for all of the cleaning processes must be attached to the treatment apparatus. Thus, the apparatus requires increased installation space and entails increased costs. Since the operation of the apparatus is stopped during the prolonged cleaning work, moreover, the operating efficiency is lowered. In view of the management of a solvent or pure water, furthermore, the wet cleaning may produce a new source of contamination if it is applied to the manufacture of VLSIs.